Many types of sensors are known for measuring the height, weight, level or volume of a material (liquid, solid or gas) in a reservoir such as bubble sensors, pressure sensors, differential pressure sensors, radar sensors, ultrasonic sensors, laser interferometric sensors, linear photo sensors, capacitive and conductive probes, plungers, floaters and scales. Many of these sensors cannot easily be used in industrial applications to monitor the rate of consumption of a material being dispensed from a reservoir when a high degree of precision is required because sensors of the required accuracy may be too expensive, too complicated or too fragile to use in the required applications.
For example, it may be desired to monitor the consumption of 1 kg of material contained in a reservoir to a precision of ±1 gram. If the consumption is monitored by measuring the level of material, the change in level will depend on the reservoir shape and could be maximized by making a very tall reservoir with a narrow cross-section. However this is not always possible due to space constraints for the equipment in the factory. In many cases the reservoir has a small compact shape and a 1 gram change in the quantity of material typically correspond to a change of 0.05 mm or less in the level of material in the reservoir. Measuring such a small change in level may require a very sophisticated and expensive level meter. Additionally, measuring the height or level of material is also vulnerable to a number of other issues: the surface of the liquid may not be flat, and at least three points should be measured (to determine the plane) in case the reservoir is inclined. Furthermore, if the liquid contains chemical agents, it may be necessary to protect the sensor from contact with the chemical to avoid damage or corrosion of the sensing apparatus.
Another type of sensor known to measure the amount of material in a reservoir is a strain meter or scale. This solution has the advantage of placing the sensor outside the reservoir, hence protected from the contents. Additionally, the measurement is then independent of the shape and orientation of the reservoir or the chemical phase of the material contained therein. One example of this type of scale is a load cell such as Vishay Tedea-Huntleigh, Single Point Aluminum, Model 1022, which is based on the Wheatstone bridge principle. This load cell is a low-cost sensor that can measure a change in strain to a high degree of precision. When this sensor is loaded with a weight, there is a vertical deformation in the sensor. The size of the deformation gives a measure of the weight of the load. Using this load cell, for example, the weight of a reservoir like the one in the previous example could be measured to a precision of ±0.01 gram if the reservoir were isolated from the rest of the dispensing apparatus.
Use of a load cell to determine the weight of the contents of a reservoir while material is being dispensed from the reservoir is described in U.S. Pat. No. 7,770,448. The solution described therein solves the problem of measuring the usage of chemicals stored in a canister regardless of chemical type or phase, but it has the drawback that a taring function is needed to allow an operator to tare the empty weight of the canister to account for differences in forces acting on the canister from the connectors that may not be present when the canister is filled prior to insertion in the dispensing apparatus.
If the position of the connectors changes during operation or if the operation is interrupted and the reservoir moved or disconnected and reconnected, the forces exerted on the reservoir by the connectors may change and a discontinuity in the continuous measurement will be produced. This discontinuity may be interpreted as an abrupt change in the measured quantity of material that could be an order of magnitude (or more) higher than the smallest change in material consumption that should be measured. If the position of the connectors changed to exert more force on the reservoir, it would be as if material had been suddenly added to the reservoir. If the position of the connectors changed such that less force was exerted on the reservoir, it would be as material has suddenly been removed from the reservoir. In the latter case, an operator monitoring consumption might erroneously think there had been a leak.
For this reason, the invention described in U.S. Pat. No. 7,770,448 includes a manual taring function; however manual retaring requires operator intervention and thus does not solve the problem if changes in the connector positions occur without the operator's knowledge. Furthermore, such intervention for retaring is tedious and requires stopping the dispensing, which leads to reduced production efficiency. If operating personnel are not attentive, the operation efficiency will be further reduced.
In some cases, the material being consumed is highly valuable. Monitoring may be desired to ensure that material is properly accounted for or does not leak during dispensation, either before the reservoir is inserted in the dispensing device, while the device is in operation, and when the device is stopped and the reservoir removed. In this case, it is important to know how much material is left in the reservoir when it is removed from the dispensing equipment.
The existing solutions do not provide a way to accurately measure consumption of material from a reservoir to a high degree of precision during dispensing, and when calibration of the measurement device cannot be reliably performed.
For these reasons it is necessary to find a solution that allows accurate measurement without requiring manual operating intervention for taring during dispensing operations or while performing maintenance procedures.